1. Field of the Invention
The present invention relates to a reproduced signal evaluation apparatus and method, a reproduction apparatus and method for an adequate evaluation of signal reproduced from a storage medium, and yet, it also relates to a recording medium and a recording method using such recording medium.
2. Description of the Related Art
In order to realize a high-density recording in a data storage apparatus in a practical manner, it is necessary to secure a certain margin against a variety of factors such as discrepancies or deviations in the process of manufacturing a storage apparatus, variation with time of operation performance of the storage apparatus, variation of temperature and discrepancies in operation performance of a recording medium used in the storage apparatus. In a signal reproducing system, if the storage apparatus incorporates a means for detecting an actual value for an evaluation of the quality of reproduced signal on a real-time basis, it is possible to operate the storage apparatus to automatically adjust the reproducing condition based on the evaluation value, whereby contributing to a substantial increase of the margin itself.
It is essential that the above evaluation value be detected with high precision and at considerable high speed. A value having significance in the evaluation of a reproduced signal is an error rate of the reproduced signal. However, in order to accomplish a stable measurement of error rate, a relatively long period of time is required. To deal with this problem, conventional practice often utilizes a jitter component of the reproduced signal as an evaluation value from an actual quality of a reproduced signal. Jitter component is a fluctuation component of the difference between a time at which a reproduced signal crosses a threshold value that constitutes a reference for encoding the reproduced signal into binary data and the time at which the reproduced signal is discriminated as binary data and it is usually expressed as a standard deviation. Originally, it is conceived as the premise that evaluation of reproduced signal via utilization of a jitter component is based on detection of a threshold value as a means for encoding the reproduced signal into binary data.
On the other hand, in recent years, as a result of the development of technology related to LSI (large-scale integrated circuit), as a practical means for encoding reproduced signal into binary signal for the sake of achieving higher recording density, such a decoder using a method of maximum likelihood typically represented by a “Viterbi” decoder for example, has become easily available. When reproducing such data aligned in arrays and recorded by way of providing correlation between them, the decoder using the method of maximum likelihood encodes them into binary signal by way of detecting the most likely array.
Nevertheless, even when encoding the reproduced signal into binary signal by applying such a “maximum likelihood” decoder cited above, in many cases, conventional practice still utilizes jitter component to determine the value from evaluation of actual quality of the reproduced signal. When utilizing this method, correlation between the actually evaluated value and the actual error rate is reduced. As a result, even after adjusting the reproducing condition based on the jitter component, such a problem is often generated, for the reproducing condition deviates from such a condition allowing minimization of the error rate.